Key Engineering Materials Vols. 629-630

Abstract: Aimed at the performance affect of high-strength super sulphate cement (SSC) paste mixed with superfine mineral admixtures, influence of microbead and silica fume replacing SSC quantity on high-strength SSC paste compression strength were studied under water-cement ratio 0.18; Hydration product morphology and phase were further compared by scanning electron microscopy and X-ray diffraction analyzer in this paper. Results show that, compared with sample HS-1, 3-day strength of HS-2 and HS-3 were increased by 5% and 10%, 28d strength basically unchanged; Furthermore, early strength of HS-7 sample slightly higher and late strength basically unchanged. SSC by adding 5% microbead and 3% silica fume (HS-11) has compressive strength 50.8MPa at 3 days and 86.1MPa at 28 days is significantly higher than other samples. Early strength of HS-11sample mainly depends on hydration reaction of SSC and particle filling effect of admixtures, later strength is due to accelerating consumption of gypsum and promoting formation of ettringite.

Abstract: In this paper, two types of modified hydrotalcites (MHT) were incorporated into cement mortars with two dosage levels (replacing 5% and 10% cement by mass). Designated testing programme including strength test, porosity test, and rapid chloride migration and diffusion test were employed to investigate the effect of modified hydrotalcites on chloride penetration in cement mortar. The results based on these tests showed the incorporation of MHT-pAB at 5% dosage in mortar specimens produced a notably improved chloride diffusion resistance with no or minor influence on the development of mechanical strength.

Abstract: Cement composites are vulnerable to harsh environments in which the chloride ions can ingress into concrete and thus cause corrosion of steel. In this study, the barrier effect of adding 2-D nanoparticles on the transport properties of cement-based materials was investigated. Graphene nanoplatelet (GNP), which comprises of a few layers of graphene stacked together, is chosen as a candidate in this study due to its impermeability and also its electrical conductivity which can be exploited for self-sensing functionality. Due to the large aspect ratio of the GNP, it is expected that the dispersion of these 2-D nanobarriers can contribute to the reduced permeability and diffusion of harmful agents. Experiments were carried out on cement mortar with 0%, 2.5%, 5.0% and 7.5% of GNP by weight of cement. The water penetration depth, chloride diffusion coefficient and chloride migration coefficient were reduced by 64%, 70% and 31% respectively with the addition of as little as 2.5% of GNP. This reduction can be attributed to the barrier effect of GNP which increases the tortuosity against water and chloride ions penetration, and also the refinement of the capillary pores which was revealed from the MIP tests. At GNP content exceeding 5%, the nanoparticles agglomerate, causing weak pockets which compromises the benefits of adding GNP to impede the ingress of fluids.

Abstract: Corrosion of rebar in reinforced concrete is a major problem affecting the integrity and loading capacity of the structures. Usually concrete pore solution provides high alkaline environment to protect steel from corrosion. However, the ingress of chloride ions or carbon dioxide would reduce the alkalinity and destroy the stable oxide film which could accelerate the corrosion process of rebar. This study was aimed to evaluate the combined effect of pH and chloride contents on corrosion behavior of rebar using simulated concrete pore solutions. Weight-loss measurement were performed to obtain the corrosion rate. Meanwhile, explore the effect of carbonation and chloride contents to Half-cell potential value on mortar and concrete. Test results show that both pH and chloride content are significant factors influencing the corrosion behavior of rebar. Higher corrosion rate was found in the rebar immersed in the solutions with smaller pH and higher chloride content.

Abstract: An experimental investigation was conducted on the durability of recycled aggregate concretes with the water to binder ratios of 0.26 and 0.60, including chloride ion penetration resistance test, freezing-thawing resistance test and water penetration resistance. Natural aggregate, recycled aggregate untreated and recycled aggregate treated by sulfuric acid solution, were employed. Results indicated that, 3 mol/L acid concentration and the 7 days soaking duration was the optimum to remove the attached mortars in recycled aggregate, and its removal rate could reach to 90.8%. Water penetration resistance, chloride ion penetration resistance and freezing-thawing resistance of concrete with 0.26 W/B was superior to that of concrete with 0.60 W/B. The more pores in the internal of concrete with 0.60 W/B could be attributed to that. Durability of recycled aggregate concrete, incorporating recycled aggregate treated by sulfuric acid solution, was improved. In particular, the improvement in recycled high strength concrete was significant.

Abstract: In this paper, permeability of concrete after loading at various stress levels, including recycled aggregate concrete (RAC) and natural aggregate concrete (NAC) (at W/B ratio of 0.586 and 0.250), is measured by means of surface infiltration experiment and chloride penetrating test to study the effect of loading size on permeability of recycled aggregate concrete. The results show that there exists a critical stress ratio value of around 0.65 to 0.75 for NAC at W/B ratio of 0.586, and among the four types of RAC, the critical stress ratio exists only in 60R50, while the water permeability coefficient increases linearly with the stress ratio for the other 3 types of RAC. The charge passed of RAC increases with the W/B ratio, and the more the recycled aggregates mixed, the higher the charge passed. As the stress ratio increases, the charge passed increases slightly in samples at W/B ratio of 0.25, while the charge passed of samples at W/B ratio of 0.586, whether NAC or RAC, increases almost linearly.

Abstract: The high contents of alkali ion and chloride ion in soda residue pollute environment. So how to treat the soda residue is an urgent problem to be solved in China. The soda residue can be used in inorganic binder stabilized materials. But the negative effect of soda residue is not studied. The impermeability, frost resistance, and chloride ion leaching of inorganic binder stabilize materials prepared by soda residue are studied. And the results as followings: (1) The permeability coefficients of cement/ mineral admixture-soda residue stabilized stone chip are very low, and they are good impermeable materials; (2) The frost resistance of cement-silica fume-soda residue stabilized stone chip is good, and silica fume is recommended to be used in cold area; (3) The mineral admixture has a better long-term immobilized effect on chloride ion than cement, and the ground granular blast-furnance slag is the best mineral admixture to immobilize the chloride ion.

Abstract: Salt frost scaling is the progressive deterioration of the concrete surface associated with freezing and thawing in the presence of a salt solution. Its major mechanism has been confirmed to be cryogenic suction that promotes continuous ice growth in the surface region. In this paper, salt frost scaling of different air-entrained concretes is investigated, alone with the room-temperature sorptivity measurement. A clear correlation is found between the two properties, which is supported by a theoretical analysis on the sorptivity. These findings explain why HPC of low w/b ratio and associated low capillary porosity has much improved scaling resistance. Similar benefits are obtained for regular w/b ratio concrete containing high cementitious replacement level of slag cement.

Abstract: High performance concrete (HPC) consisting of low water-binder (w/b) ratio and supplementary cementitious materials (SCM) is more prone to shrinkage cracking if subjected to external deformation restraint. The effectiveness of using lightweight fine aggregate (LWA) for autogenous shrinkage reduction is being studied along with implications on salt frost durability. HPC consisting of 0.33 w/b ratio and cementitious replacement level up to 50% by slag cement and natural sand replacement level by LWA of up to 50% is investigated. Results indicate that these concretes exhibit excellent salt frost resistance provided the HPC is sufficiently air entrained. The mitigation of autogenous shrinkage by LWA is analyzed by comparing the spacing of LWA particles in cement paste and the flow distance of retained moisture in LWA to the adjacent paste.

Abstract: High volume utilization of industrial wastes and by products is the solution for high disposal coast. The anti-frost of high performance concrete is a key factor for safe utilization of concrete structure containing industrial wastes under severe environment. In this paper, to understand the property on anti-frost of high performance containing ground granulated blast furnace slag (GGBFS) under cold marine environment. Some comparison studies were conducted on plain concrete by rapid freeze-thaw cycle test. During the rapid freeze-thaw cycle test, the mass loss and relative elastic modulus were measured regularly at the prescribed conditioning ages. The development of microstructure in concrete was analyzed through scanning electron microscopy (SEM) and X-ray diffraction (XRD). The rapid freeze-thaw cycle test results show that the plain concrete was destroyed severely at 150 freeze-thaw cycles. After 225 freeze-thaw cycles, the mass loss and the relative dynamic modulus of elasticity of GGBFS concrete decrease 1.3% and 26.11%, respectively, that indicates that GGBFS significantly improve the anti-frost performance of concrete; The addition of GGBFS can accelerate the cement hydration reaction, promote more Ca (OH)₂ crystals shift to C-S-H gel and help to increase the density of the micro-structure of concrete, which can prevent the formation of micro-cracks and suppress the propagation of cracks and thus effectively improve the durability of concrete. KEY WORDS: GGBFS; freeze-thaw durability; microstructure; scanning electron microscopy; X-ray diffraction.